Page 446 - High Power Laser Handbook

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414 Fi b er L a s er s Intr oduction to Optical Fiber Lasers 415

(EDFAs) were the dominant fiber technology research topic in the late
8,9
1980s. The second technology was the availability of pump diodes
that could be used in EDFAs to provide robust, compact devices for
use in demanding telecommunication applications, such as submarine
systems. 10,11 The ability of optical fiber amplifiers to replace electronic
repeaters and consequently to eliminate the need for optical-to-
electronic and electronic-to-optical conversion were easy to see. This
was especially true when the wavelength-transparent nature of opti-
cal fiber amplifiers was considered; wavelength-division-multiplex-
ing transmission systems were seen as the path to meet the increasing
bandwidth demands for the new digital data traffic beyond the tradi-
tional analogue traffic. Both single-mode erbium-doped fibers and
pump diodes are derived from technologies that have been well
known and field tested in the telecommunication industry. The matu-
rity of the key technologies, in combination with an increasing
demand in data traffic, was key to the rapid development and subse-
quent deployment of EDFAs in the early 1990s.
The development of EDFAs, which was spurred by important
telecommunication market demands and backed by the immense
resources of the telecommunication industry, quickly led to the wide
availability of knowledge, components, technologies, and equipment
relevant to the development of optical fiber lasers. This, in turn, led to
an extensive amount of research being conducted on rare-earth-
doped fiber lasers covering continuous wave (CW) lasers, Q-switched
lasers, mode-locked lasers, upconversion lasers, and single-frequency
lasers in the late 1980s and early 1990s. For the majority of the 1990s,
the highest-power single-mode optical fiber lasers were usually
pumped by gas lasers or solid-state lasers, which made them imprac-
tical for commercial applications. The average powers from most
fiber lasers pumped by single-mode pump diodes were too low for
serious industry applications.
Another important development in the late 1980s led to the
eventual development of single-mode optical fiber lasers with
average powers well above the subwatt levels available from single-
mode pump diodes. Cladding pumping was initially studied to
enable the use of much higher powers available from low-bright-
ness multimode single-emitter diodes, 12,13 which were being devel-
oped for pumping solid-state lasers, leading to more efficient and
reliable high-power solid-state lasers. A double-clad fiber used in
cladding pumping has a small rare-earth-doped single-mode core
embedded in a much larger multimode pump guide. This configu-
ration effectively behaves like a brightness converter, which allows
conversion of highly multimode low-brightness pump light to a
single-mode high-brightness laser beam guided in the rare-earth-
doped single-mode core. Because of the limited brightness and low
packing density of multiemitter diodes or diode arrays, some form
of beam-shaping optics is often used to further maximize the pump
power that can be coupled into a double-clad fiber. The development